Batteries of various configurations are used to generate electric current at a certain voltage through conversion of chemical energy into electrical energy. Such batteries typically include plural electrodes and an electrolyte individually having respective reactive masses. The chemical energy is converted into electric energy through a reaction of the reactive masses of the electrodes and/or the electrolyte.
More specifically, batteries typically produce electrical energy through a chemical reaction between the electrolyte solution and at least one electrode in contact with the electrolyte solution, or through a chemical reaction between two electrodes through the electrolyte solution. As the chemical reaction takes place, mass is transferred or exchanged between an electrode and the electrolyte, or between electrodes through the electrolyte. The chemical reaction takes place during the discharge of the battery to create the current that is supplied to a load during battery discharge. Reversed chemical reactions take place during a recharging process.
The weight, density and volume of the reacting electrode or electrodes and/or electrolyte changes during the chemical reactions. The respective volumes of the electrodes and electrolyte can gradually change from first extreme values corresponding to volumes with the battery in a fully charged condition to second extreme values corresponding to volumes with the battery in a functionally discharged condition. The battery is depleted or discharged when the reactive mass of an electrode or the electrolyte is depleted.
Volumetric changes of one or both electrodes and/or the electrolyte of a battery usually occur in a substantially linear fashion as the battery discharges from its substantially fully charged condition to its functionally discharged condition, or vice-versa. If the battery is a rechargeable battery (i.e., secondary battery), the chemical reaction is reversed during recharging. During recharging, volumetric changes are reversed and the respective volumes of the battery components substantially return to the initial respective values corresponding to the battery being fully charged.
More specifically, the chemical reaction producing electricity typically changes the chemical composition of one or more of the internal components of the battery. New chemicals are usually produced as a result of the reaction and the new chemicals typically have different specific volumes. Accordingly, the total volume of internal battery components (i.e., electrodes, electrolyte) changes with every discharging or recharging increment of the battery.
In a discharged condition, a battery is no longer able to supply adequate voltage and current. Primary batteries are typically not rechargeable and are discarded. Secondary batteries are rechargeable by application of an electrical current to the terminals of the battery. In particular, the chemical energy from the reactions between the electrodes and the electrolyte that is spent producing electricity can be replenished by reversing the chemical reactions on the electrodes during a recharging process. The recharging process involves passing an electric current from another source through the battery in the reverse direction.
Primary batteries are usually manufactured for supplying limited amounts of electricity and are used in a wide range of equipment. Secondary batteries are capable of delivering considerable amounts of electricity with high currents and are also used in a wide variety of applications (e.g., backup power supplies, main power source in some electric vehicles).
The amount of electric energy that can be extracted from a battery depends on its electric potential, expressed in Volts, and the number of Coulombs (electric current intensity times its duration expressed in Ampere-hours) that the battery can sustain.
One battery configuration includes lead-acid batteries. These batteries are typically utilized in strong current applications, such as automotive applications. Lead-acid batteries usually include a plurality of cells which are linked with one another. Such batteries are rechargeable by providing a recharging current into the batteries.
Prior art devices have been developed to measure and/or display the amount of remaining electric energy, or a fraction or percentage of the initial total electric energy, known as state of charge, of an electric battery at various stages of the useful life of the battery. For example, U.S. Pat. Nos. 5,567,541 and 5,580,675, both incorporated herein by reference, provide a history of many developments in this field. However, there remains a need to provide a state of charge indicator for batteries having improved features over the existing prior art devices.